Chalk Group Lithostratigraphy: Central & Northern North Sea (UK Sector) - Tor Formation

The following details are based on the account of Johnson & Lott (1993)

The formation comprises relatively hard, pure, chalky limestones, with locally interbedded softer argillaceous chalky limestones. Slumped and chaotically bedded intervals of the formation have higher porosity and are important petroleum reservoirs in the North Sea. Although generally less than 150 m thick, this widespread formation reaches more than 250 m thick in basinal areas, and locally exceeds 500 m in some sections.

The base of the formation is lithologically marked by the sharp downward change from hard, pure chalky limestones to the relatively softer argillaceous limestones of the underlying Mackerel Formation. Geophysically this corresponds with a downward increase in gamma values and reduction in sonic velocity on geophysical logs.

The top of the formation is marked by the downward lithological change from argillaceous chalky limestones of the overlying Ekofisk Formation to purer limestones. Geophysically this boundary is marked by a reduction in gamma log values.

Age: generally Early Maastrichtian, but locally, latest Campanian to latest Maastrichtian

Biozonation:

Foraminifera:

• BGS Biozones: inferred to equate with BGS biozones 25 (pars) - 27

• BGS Biomarkers: contains BGS Biomarkers 1 & 2

Dinoflagellates: equates with BGS zones 17-18

'References

JOHNSON, H & LOTT, G K. 1993. 2. Cretaceous of the Central and Northern North Sea. In, KNOX, R W O'B & CORDEY, W G. Lithostratigraphic nomenclature of the UK North Sea. (British Geological Survey)